The present invention relates to an apparatus for lighting a discharge lamp in such a way as to prolong the life of the discharge lamp by adding a high-frequency operation as occasion arises to the low-frequency operation of a polarity switching circuit for use in supplying power to the discharge lamp, normally with a low-frequency AC square wave, to suppress an overcurrent flowing into the discharge lamp and to prevent the electrodes from abrasion when the discharge lamp such as a high pressure sodium lamp, a metallic halide lamp, a high pressure mercury lamp or the like is started.
An apparatus of the sort arranged as shown in FIG. 8 has heretofore been in use for lighting a discharge lamp such as a high pressure sodium lamp, a metallic halide lamp, a high pressure mercury lamp or the like.
In FIG. 8, numeral 1 denotes a discharge lamp; 2, a input power supply; and 3, an inverter circuit for boosting the source voltage of the input power supply 2 up to a voltage what is required for the discharge lamp 1. Further, numeral 4 denotes a rectifying smoothing circuit for rectifying and smoothing the output of the invertor circuit 3, which includes a rectifier diode D and a smoothing capacitor C.
Further, numeral 5 denotes a lamp current detection resistor for detecting a lamp current flowing into the discharge lamp 1; 6, 7 lamp voltage detection resistors for detecting a lamp voltage to be applied to the discharge lamp 1; 8, a feedback control circuit for computing the power required for the discharge lamp 1 according to the lamp current detected by the resistor 5 and the lamp voltage detected by the resistors 6, 7 to cause the power thus required to be output by controlling the invertor circuit 3 in the feedback mode.
Further, numeral 9 denotes a polarity switching circuit for supplying power with a low-frequency AC square wave to stabilize the discharge arc while preventing the acoustic resonant phenomenon of the discharge lamp 1 and to prevent the cataphoretic phenomenon thereof resulting in color separation in the light emitting portion.
The polarity switching circuit 9 is formed into a full bridge type having four switching elements Q1, Q2, Q3, Q4 and four parasitic diodes D1, D2, D3, D4 connected to the respective switching elements Q1, Q2, Q3, Q4. The pairs of switching elements Q1, Q4 and Q2, Q3 are driven by a low-frequency drive circuit 10 in such a manner that they are alternately turned on and off, whereby the polarity applied to the discharge lamp 1 is inverted.
Further, numeral 11 denotes an ignitor for starting the discharge lamp 1 by superposing a high-voltage pulse thereon, the ignitor including a pulse transformer 12 and a trigger circuit 13.
When the discharge lamp 1 is started, the inverter circuit 3 boosts the source voltage of the input power supply 2 up to a voltage that is required to start the discharge lamp 1, thus causing the prescribed voltage (e.g., about 320 V) to be generated. The rectifying smoothing circuit 4 rectifies And smoothes the output of the invertor circuit 3 and then applies to the discharge lamp 1 the resulting output via the polarity switching circuit 9 and the ignitor 11.
The pair of switching elements Q1, Q4 or Q2, Q3 of the polarity switching circuit 9 is turned on, so that the starting voltage is applied to the discharge lamp 1. At this time, the lamp impedance Z1a of the discharge lamp 1 is approximately infinity (Z1a.congruent..infin.).
When the ignitor 11 superposes the high-voltage pulse on the starting voltage and applies the resulting voltage to the discharge lamp 1, breakdown occurs in the discharge lamp 1, thus causing an arc discharge through a glow discharge therein. Since the vapor pressure in the light emitting tube of the discharge lamp 1 is low at this time, the lamp impedance Z1a rapidly drops down. The charge stored in the smoothing capacitor C is rapidly discharged as the lamp impedance Z1a sharply changes and the lamp current flows into the discharge lamp 1 in the form of an overcurrent as shown by a dotted line of FIG. 4.
At this time, the feedback control circuit 8 detects the excessive current and steeply decreases the output of the inverter circuit 3 so as to suppress the overcurrent flowing into the discharge lamp 1. However, the charge stored in the smoothing capacitor C ultimately flows through the polarity switching circuit 9, the ignitor 11 and the discharge lamp 1 as an instantaneous overcurrent, irrespective of the output of the inverter circuit 3 because the discharge route exists only on the side of the discharge lamp 1.
For this reason, sputtering has been induced in the electrodes of the discharge lamp 1, which causes the electrodes to abrade. Moreover, broken particle of the electrodes due to the sputtering stick to the inner wall of the light emitting tube of the discharge lamp 1, thus badly affecting the life of the discharge lamp 1. In other words, the electrodes abrasion as time elapses and the electrode-to-electrode distance increases as shown in FIG. 5. The drawback among others is that the life of the discharge lamp 1 is shortened.
FIG. 5 shows electrode-to-electrode distance curves when a 35W metallic halide lamp used as the discharge lamp 1 is continuously repeatedly held "on" for 5 minutes and "off" for 10 seconds in cycles.